Flue gas recirculation (FGR) is a technology that introduces part of the flue gas at the tail of the boiler back to the combustion area, mixes it with air or fuel, and then participates in combustion. It is widely used in industrial boilers, gas turbines and other combustion equipment. Its main functions are as follows:
1. Reduce nitrogen oxide (NOₓ) emissions
This is the core role of flue gas recirculation. The principle is as follows:
Reducing combustion temperature:
Flue gas contains a large amount of inert gas (such as N₂, CO₂, etc.), which has a large specific heat capacity. After being introduced into the combustion area, it will absorb part of the heat and reduce the combustion temperature. The generation of NOₓ (especially thermal NOₓ) is closely related to temperature. When the combustion temperature is lower than 1300℃, the amount of NOₓ generated will be significantly reduced.
Diluting oxygen concentration:
Flue gas recirculation will dilute the oxygen (O₂) concentration in the combustion area, slow down the combustion reaction rate, and make the combustion process tend to "oxygen-poor combustion", thereby inhibiting the generation of NOₓ.
Changing the combustion atmosphere:
Compositions such as CO₂ and water vapor (H₂O) in the flue gas will participate in the combustion reaction and inhibit the generation path of NOₓ through chemical equilibrium (such as reducing the oxygen partial pressure, participating in the reduction reaction of nitrogen oxides, etc.).
2. Improving combustion efficiency and stability
Optimizing fuel-air mixing:
Flue gas recirculation can enhance the turbulence in the combustion area, promote the uniform mixing of fuel and air, make the combustion more complete, and reduce the generation of incomplete combustion products such as unburned carbon particles and carbon monoxide (CO).
Stable low-load combustion:
When the boiler is running at low load, the combustion temperature and flame stability are prone to decrease. After the flue gas is introduced, the combustion temperature can be maintained by adjusting the recirculation rate (the proportion of flue gas volume to the total flue gas volume), avoiding flameout and expanding the stable operating load range of the equipment.
3. Adjusting the boiler temperature distribution
Balanced heat load on the heating surface:
For large boilers, excessively high temperatures in the combustion area may cause overheating or slagging of the heating surface. Flue gas recirculation can make the temperature field in the furnace more uniform by reducing the center temperature of the flame and adjusting the distribution of flue gas flow, thereby reducing local overheating problems and extending the service life of the heating surface.
Control steam temperature:
In some boiler systems, the flue gas flow and temperature of the superheater or reheater can be changed by adjusting the flue gas recirculation volume, thereby indirectly controlling the steam temperature and reducing the dependence on auxiliary adjustment measures such as water spray cooling.
IV. Other functions
Reducing pollutant co-emissions:
In addition to NOₓ, flue gas recirculation may also have an indirect impact on the emission of other pollutants (such as sulfur dioxide SO₂, particulate matter, etc.). For example, lowering the combustion temperature may reduce the gasification of certain volatile heavy metals, thereby reducing the content of heavy metals in particulate matter.
Adapt to fuel changes:
When fuel characteristics (such as calorific value, sulfur content, volatile matter) fluctuate, flue gas recirculation can enhance the adaptability of the equipment to fuel changes by adjusting the combustion conditions, ensuring the stability of combustion efficiency and pollutant emissions.
Application scenarios and precautions
Main applications:
It is widely used for NOₓ emission reduction in power plant boilers, industrial kilns, gas turbines, incinerators and other equipment, especially when used with low-nitrogen burners.
Key parameters:
The flue gas recirculation rate is usually 5%~20%. If it is too high, the combustion temperature will be too low, affecting the efficiency; if it is too low, the NOₓ emission reduction effect is not obvious, and it needs to be optimized according to the fuel type and equipment characteristics.
Limitations:
It may increase the energy consumption of the induced draft fan, and for sulfur-containing flue gas, the low temperature area may aggravate the corrosion of the equipment (anti-corrosion measures are required).
In short, flue gas recirculation achieves NOₓ emission reduction through the dual mechanisms of "temperature control" and "atmosphere adjustment", while optimizing the combustion efficiency and equipment operation stability. It is one of the important technologies for pollution control of current combustion equipment.
Flue gas recirculation (FGR) is a technology that introduces part of the flue gas at the tail of the boiler back to the combustion area, mixes it with air or fuel, and then participates in combustion. It is widely used in industrial boilers, gas turbines and other combustion equipment. Its main functions are as follows:
1. Reduce nitrogen oxide (NOₓ) emissions
This is the core role of flue gas recirculation. The principle is as follows:
Reducing combustion temperature:
Flue gas contains a large amount of inert gas (such as N₂, CO₂, etc.), which has a large specific heat capacity. After being introduced into the combustion area, it will absorb part of the heat and reduce the combustion temperature. The generation of NOₓ (especially thermal NOₓ) is closely related to temperature. When the combustion temperature is lower than 1300℃, the amount of NOₓ generated will be significantly reduced.
Diluting oxygen concentration:
Flue gas recirculation will dilute the oxygen (O₂) concentration in the combustion area, slow down the combustion reaction rate, and make the combustion process tend to "oxygen-poor combustion", thereby inhibiting the generation of NOₓ.
Changing the combustion atmosphere:
Compositions such as CO₂ and water vapor (H₂O) in the flue gas will participate in the combustion reaction and inhibit the generation path of NOₓ through chemical equilibrium (such as reducing the oxygen partial pressure, participating in the reduction reaction of nitrogen oxides, etc.).
2. Improving combustion efficiency and stability
Optimizing fuel-air mixing:
Flue gas recirculation can enhance the turbulence in the combustion area, promote the uniform mixing of fuel and air, make the combustion more complete, and reduce the generation of incomplete combustion products such as unburned carbon particles and carbon monoxide (CO).
Stable low-load combustion:
When the boiler is running at low load, the combustion temperature and flame stability are prone to decrease. After the flue gas is introduced, the combustion temperature can be maintained by adjusting the recirculation rate (the proportion of flue gas volume to the total flue gas volume), avoiding flameout and expanding the stable operating load range of the equipment.
3. Adjusting the boiler temperature distribution
Balanced heat load on the heating surface:
For large boilers, excessively high temperatures in the combustion area may cause overheating or slagging of the heating surface. Flue gas recirculation can make the temperature field in the furnace more uniform by reducing the center temperature of the flame and adjusting the distribution of flue gas flow, thereby reducing local overheating problems and extending the service life of the heating surface.
Control steam temperature:
In some boiler systems, the flue gas flow and temperature of the superheater or reheater can be changed by adjusting the flue gas recirculation volume, thereby indirectly controlling the steam temperature and reducing the dependence on auxiliary adjustment measures such as water spray cooling.
IV. Other functions
Reducing pollutant co-emissions:
In addition to NOₓ, flue gas recirculation may also have an indirect impact on the emission of other pollutants (such as sulfur dioxide SO₂, particulate matter, etc.). For example, lowering the combustion temperature may reduce the gasification of certain volatile heavy metals, thereby reducing the content of heavy metals in particulate matter.
Adapt to fuel changes:
When fuel characteristics (such as calorific value, sulfur content, volatile matter) fluctuate, flue gas recirculation can enhance the adaptability of the equipment to fuel changes by adjusting the combustion conditions, ensuring the stability of combustion efficiency and pollutant emissions.
Application scenarios and precautions
Main applications:
It is widely used for NOₓ emission reduction in power plant boilers, industrial kilns, gas turbines, incinerators and other equipment, especially when used with low-nitrogen burners.
Key parameters:
The flue gas recirculation rate is usually 5%~20%. If it is too high, the combustion temperature will be too low, affecting the efficiency; if it is too low, the NOₓ emission reduction effect is not obvious, and it needs to be optimized according to the fuel type and equipment characteristics.
Limitations:
It may increase the energy consumption of the induced draft fan, and for sulfur-containing flue gas, the low temperature area may aggravate the corrosion of the equipment (anti-corrosion measures are required).
In short, flue gas recirculation achieves NOₓ emission reduction through the dual mechanisms of "temperature control" and "atmosphere adjustment", while optimizing the combustion efficiency and equipment operation stability. It is one of the important technologies for pollution control of current combustion equipment.